Current Transformers Selection Guide

Low voltage current transformers (CTs)

Determination of the customer’s needs

Electrical characteristics of the primary circuits suivant Norme IEC

The primary circuits of the current transformer must withstand the constraints related to the medium voltage network to which it is connected.

Rated frequency

This is the frequency of the installation.

A CT defined for 50 Hz can be installed on a 60 Hz network with the same level of accuracy. However, the opposite is not true. For a non-referenced unit, it is vital to indicate the rated frequency on the order from.

Rated voltage of the primary circuit (Upn)

General case:

Insulation level continuity for the whole installation will be ensured if the rated voltage of the CT used is the rated voltage of the installation. The rated voltage determines the insulation level of the equipment. Generally we choose the rated voltage based on the duty voltage, Us, according to the following table:

Rated voltage of the primary circuit - General case

Specific case:
If the CT is installed on a bushing or a cable providing insulation, the CT can be LV ring type.

Primary service current (Ips)

Knowledge about the primary service current will enable us to determine the rated primary current for the CT taking into account any eventual derating.

The service current depends on the power traversing the primary windings of the CT.

If:
S = apparent power in VA
Ups = primary service voltage in V
P = active power of the motor in W
Q = reactive power of the capacitors in VAR
Ips = primary service current in Amp

We will have :

Incoming cubicle:

Generator incomer:

Transformer feeder:

Motor feeder:

η = efficiency of the motor

If you do not know exact values for j and h as a first approximation, you can assume that: cos j = 0.8 ; h = 0.8

Capacitor feeder:

1.3 is a de-rating factor of 30% which compensates for heat-up due to harmonics in the capacitors.

Bus tie:

The Ips current in the CT is the highest permanent current that can circulate in the connection.

Measurement of insulation resistance (IR)

Megger MIT1020 10-kV insulation resistance testers are all designed specifically to assist the user with the testing and maintenance of high voltage equipment.

Introduction

The measurement of insulation resistance is a common routine test performed on all types of electrical wires and cables. As a production test, this test is often used as a customer acceptance test, with minimum insulation resistance per unit length often specified by the customer. The results obtained from IR Test are not intended to be useful in finding localized defects in the insulation as in a true HIPOT test, but rather give information on the quality of the bulk material used as the insulation.

Even when not required by the end customer, many wire and cable manufacturers use the insulation resistance test to track their insulation manufacturing processes, and spot developing problems before process variables drift outside of allowed limits.

Selection of IR Testers (Megger):

Insulation testers with test voltage of 500, 1000, 2500 and 5000 V are available. The recommended ratings of the insulation testers are given below:

Voltage Level	IR Tester
650V	500V DC
1.1KV	1KV DC
3.3KV	2.5KV DC
66Kv and Above	5KV DC

 Test Voltage for Meggering:

When AC Voltage is used, The Rule of Thumb is:
Test Voltage (A.C) = (2X Name Plate Voltage) +1000.

When DC Voltage is used (Most used in All Megger)
Test Voltage (D.C) = (2X Name Plate Voltage).

Equipment / Cable Rating	DC Test Voltage
24V To 50V	50V To 100V
50V To 100V	100V To 250V
100V To 240V	250V To 500V
440V To 550V	500V To 1000V
2400V	1000V To 2500V
4100V	1000V To 5000V

Measurement Range of Megger:

Test voltage	Measurement Range
250V DC	0MΩ to 250GΩ
500V DC	0MΩ to 500GΩ
1KV DC	0MΩ to 1TΩ
2.5KV DC	0MΩ to 2.5TΩ
5KV DC	0MΩ to 5TΩ

Precaution while Meggering

Before Meggering:

Make sure that all connections in the test circuit are tight. Test the megger before use, whether it gives INFINITY value when not connected, and ZERO when the two terminals are connected together and the handle is rotated.

During Meggering:

Make sure when testing for earth, that the far end of the conductor is not touching, otherwise the test will show faulty insulation when such is not actually the case.

Make sure that the earth used when testing for earth and open circuits is a good one otherwise the test will give wrong information. Spare conductors should not be meggered when other working conductors of the same cable are connected to the respective circuits.

After completion of cable Meggering:

• Ensure that all conductors have been reconnected properly.
• Test the functions of Points, Tracks & Signals connected through the cable for their correct response.
• In case of signals, aspect should be verified personally.
• In case of points, verify positions at site. Check whether any polarity of any feed taken through the cable has got earthed inadvertently.

Safety Requirements for Meggering:

• All equipment under test MUST be disconnected and isolated.
• Equipment should be discharged (shunted or shorted out) for at least as long as the test voltage was applied in order to be absolutely safe for the person conducting the test.
• Never use Megger in an explosive atmosphere.
• Make sure all switches are blocked out and cable ends marked properly for safety.
• Cable ends to be isolated shall be disconnected from the supply and protected from contact to supply, or ground, or accidental contact.
• Erection of safety barriers with warning signs, and an open communication channel between testing personnel.
• Do not megger when humidity is more than 70 %.
• Good Insulation: Megger reading increases first then remain constant.
• Bad Insulation: Megger reading increases first and then decreases.
• Expected IR value gets on Temp. 20 to 30 decree centigrade.
• If above temperature reduces by 10 degree centigrade, IR values will increased by two times.
• If above temperature increased by 70 degree centigrade IR values decreases by 700 times.

How to use Megger

Meggers is equipped with three connection Line Terminal (L), Earth Terminal (E) and Guard Terminal (G).

Megger connections

Resistance is measured between the Line and Earth terminals, where current will travel through coil 1. The “Guard” terminal is provided for special testing situations where one resistance must be isolated from another. Let’s us check one situation where the insulation resistance is to be tested in a two-wire cable.

To measure insulation resistance from a conductor to the outside of the cable, we need to connect the “Line” lead of the megger to one of the conductors and connect the “Earth” lead of the megger to a wire wrapped around the sheath of the cable.

Megger configuration

In this configuration the Megger should read the resistance between one conductor and the outside sheath.

We want to measure Resistance between Conductor- 2 to sheaths but actually megger measure resistance in parallel with the series combination of conductor-to-conductor resistance (R_c1-c2) and the first conductor to the sheath (R_c1-s).

If we don’t care about this fact, we can proceed with the test as configured. If we desire to measure only the resistance between the second conductor and the sheath (R_c2-s), then we need to use the megger’s “Guard” terminal.

Megger - Connecting guard terminal

Connecting the “Guard” terminal to the first conductor places the two conductors at almost equal potential.

With little or no voltage between them, the insulation resistance is nearly infinite, and thus there will be no current between the two conductors. Consequently, the Megger’s resistance indication will be based exclusively on the current through the second conductor’s insulation, through the cable sheath, and to the wire wrapped around, not the current leaking through the first conductor’s insulation.

The guard terminal (if fitted) acts as a shunt to remove the connected element from the measurement. In other words, it allows you to be selective in evaluating certain specific components in a large piece of electrical equipment. For example consider a two core cable with a sheath.

As the diagram below shows there are three resistances to be considered.

Meggering wiring

If we measure between core B and sheath without a connection to the guard terminal some current will pass from B to A and from A to the sheath. Our measurement would be low. By connecting the guard terminal to A the two cable cores will be at very nearly the same potential and thus the shunting effect is eliminated.

Motor Service Factor (SF) Defined By NEMA

Motor Service Factor (SF) Defined By NEMA

Permissible horsepower loading

Motor Service Factor (SF) is the percentage of overloading the motor can handle for short periods when operating normally within the correct voltage tolerances. This is practical as it gives you some ‘fudge‘ in estimating horsepower needs and actual running horsepower requirements.

It also allows for cooler winding temperatures at rated load, protects against intermittent heat rises, and helps to offset low or unbalanced line voltages.

BALDOR Open Drip Proof C-Face Foot Mounted motor - 1/3Hp-100Hp NEMA 56C-404TC

For example, the standard SF for open drip-proof (ODP) motors is 1.15. This means that a 10-hp motor with a 1.15 SF could provide 11.5 hp when required for short-term use. Some fractional horsepower motors have higher service factors, such as 1.25, 1.35, and even 1.50.

NEMA defines service factor as a multiplier, when applied to the rated horsepower, indicates a permissible horsepower loading, which may be carried under the conditions specified for the service factor at rated voltage and frequency.

This service factor can be used for the following:

1. To accommodate inaccuracy in predicting intermittent system horsepower needs.
2. To lengthen insulation life by lowering the winding temperature at rated load.
3. To handle intermittent or occasional overloads.
4. To allow occasionally for ambient above 40°C.
5. To compensate for low or unbalanced supply voltages.

NEMA does add some cautions, however, when discussing the service factor:

1. Operation at service factor load for extended periods will usually reduce the motor speed, life and efficiency.
2. Motors may not provide adequate starting and pull-out torques, and incorrect starter/overload sizing is possible. This in turn affects the overall life span of the motor.
3. Do not rely on the service factor capability to carry the load on a continuous basis.
4. The service factor was established for operation at rated voltage, frequency, ambient and sea level conditions.

Most motors have a duty factor of 1.15 for open motors and 1.0 for totally closed motors.

Traditionally, totally enclosed fan cooled (TEFC) motors had an SF of 1.0, but most manufacturers now offer TEFC motors with service factors of 1.15, the same as on ODP motors. Most hazardous location motors are made with an SF of 1.0, but some specialized units are available for Class I applications with a service factor of 1.15.

The service factor is required to appear on the nameplate only if it is higher than 1.0.

NEC Requirements for Emergency Systems

NEC Requirements for Emergency Systems (on photo Automatic Transfer Switch for Emergency Systems 240V 150A 3p)

Introduction

Emergency systems are generally installed in buildings that are or can be occupied by 1000 or more persons or are more than 75 ft high.

These are buildings where artificial illumination is required for safe exiting and for panic control. Examples are hotels, theaters, airports, railroad stations, sports arenas, department stores, and hospitals.

Emergency systems are designed to power exit lighting, fire detection and alarm systems, elevators, fire pumps, and public safety communications systems. They might also power ventilation systems considered essential to preserving health and life, or industrial processes where power interruption would result in hazards to life or injury.

NEC 2012, Article 700, “Emergency Systems,” covers electrical safety in the installation, operation, and maintenance of emergency systems. These consist of “circuits and equipment intended to supply, distribute, and control electricity for illumination, power or both, to vital facilities when the normal electrical supply or system is interrupted”.

These are “systems legally required and classed as emergency by municipal, state, federal, or others codes, or by any governmental agency having jurisdiction”.

These systems are intended to automatically supply illumination, power, or both to designated areas and equipment in the event of failure of the normal supply or in the event of accident to elements of a system intended to supply, distribute, and control power and illumination essential to human life.”

The general subjects covered in Article 700 include:

• Tests and maintenance of approved emergency system equipment
• Capacity and rating of emergency system equipment
• Power transfer equipment, including automatic transfer switches
• Signals and signs for emergency systems

The circuit wiring provisions of Article 700 include:

• Identification of boxes, enclosures, transfer switches, generators, etc.
• Wiring independence and exceptions
• Fire protection for high-occupancy and high-rise buildings

The section on sources of power gives the response-time requirements for the restoration of emergency lighting, emergency power, or both as “not to exceed 10 seconds” for the specific classes of buildings stated previously.

In selecting the emergency source of power, consideration must be given to the occupancyand type of service rendered in those buildings.

The occupancy classes are given as (1) assembly, (2) educational, (3)residential, (4) detention and correctional, (5) business, and (6) mercantile.

Article 700 requires that power sources be installed in rooms protected by approved automatic fire suppression systems (sprinklers, CO2systems, etc.) or in spaces with a 1-hr burn rating. (Fire can surround or be adjacent to the room for at least 1 hr before its fire-resistant integrity is lost and its contents begin to ignite spontaneously.)

The four emergency power systems approved by Article 700 are:

• Storage batteries (rechargeable)
• Generator sets
• Uninterruptible power supplies (UPS)
• Separate services (alternate outside utility or inside generation) in accordance with NEC Article 230

The section on emergency system circuits for lighting and power covers:

• Approved loads on emergency branch circuits
• Emergency illumination
• Circuits for emergency lighting
• Circuits for emergency power

The section on emergency control lighting circuits covers:

• Switch requirements
• Switch location
• Exterior lights

The section on overcurrent protection covers accessibility of branch-circuit overcurrent devices (fuses and circuit breakers) and ground-fault protection of equipment.

Identifying The Primary And Secondary Phasor Polarities Of Transformer – Polarity Test

CPC 100 - Universal testing device for electrical diagnostics on transformers, current transformers, voltage transformers, grounding systems, lines and cables, and circuit breakers (photo by www.omicron.at)

Polarity Detection

This is needed for identifying the primary and secondary phasor polarities. It is a must for poly phase connections. Both a.c. and d.c methods can be used for detecting the polarities of the induced emfs.

The dot method is used to indicate the polarities.

The transformer is connected to a low voltage a.c. source with the connections made as shown in the Figure 1 (a). A supply voltage Vs is applied to the primary and the readings of the voltmeters V₁, V₂ and V₃ are noted. V₁ : V₂ gives the turns ratio.

If V₃ reads V₁−V₂ then assumed dot locations are correct (for the connection shown).

Figure 1 - Transformer polarity test scheme

The beginning and end of the primary and secondary may then be marked by A₁ − A₂ and a₁− a₂ respectively. If the voltage rises from A₁ to A₂ in the primary, at any instant it does so from a₁ to a₂ in the secondary.

If more secondary terminals are present due to taps taken from the windings they can be labeled as a₃, a₄, a₅, a₆. It is the voltage rising from smaller number towards larger ones in each winding. The same thing holds good if more secondaries are present.

Figure 1 (b) shows the d.c. method of testing the polarity. When the switch S is closed if the secondary voltage shows a positive reading, with a moving coil meter, the assumed polarity is correct. If the meter kicks back the assumed polarity is wrong.